This invention relates to a subsea riser manifold system for handling oil and/or gas production from offshore wells. In particular, it provides a structure for supporting a subsea riser on a marine floor base.
This invention relates to the production of hydrocarbon fluids from subaqueous formations utilizing a system of submerged wellheads and a product gathering network. Recent developments in the offshore oil and gas industry extend production to undersea areas, such as the outer fringes of the continental shelves and the continental slopes. A submarine production system is believed to be the most practical method of reaching the subaqueous deposits. Although hydrocarbons are the main concern at this time, it is contemplated that subaqueous deposits of sulfur and other minerals can be produced from beneath the seas. While bottom-supported permanent surface installations have proved to be economically and technologically feasible in comparatively shallow waters, in deeper waters, such as several hundred to several thousand meters, utilization of such surface installations must be limited to very special situations. Installations extending above the water surface are also disadvantageous even in shallower water where there are adverse surface conditions, as in areas where the bottom-supported structure of above-surface production platforms are subject to ice loading.
Subsea production and gathering systems are feasible for installing wellheads or well clusters at multiple locations on a marine floor area. Flowlines for production fluids, injection fluids, hydraulic controls, etc. can be laid on the marine floor from remote locations to a central point for connection to a production riser, which connects a manifold system to a surface facility for processing. Habitable satellites can be maintained adjacent the wellhead or manifold structures for operating and maintenance personnel, as disclosed in U.S. Pat. No. 3,520,358 (Brooks et al). One such satellite may be a subsea atmospheric riser manifold (SARM), which contains a fluid handling system for operatively connecting a plurality of flowlines to a production riser. Such a manned system could have a central hull chamber enclosing the manifold piping, valves, etc. and a control room for sustaining life in the extreme environmental conditions of deepwater. In order to enclose a multi-well manifold system, such a manifold chamber would be necessarily large and would require great vessel integrity to withstand the deepwater hydrostatic pressure, equivalent to many atmospheres exterior pressure. The SARM system should be capable of supporting human life over long periods, which requires internal pressures at or near atmospheric.
Riser manifold systems have not been successful in large production gathering networks due to the extreme conditions for connecting a heavy duty production riser with a large multi-well subsea manifold system. Recent advances in production riser design (e.g. U.S. Pat. No. 4,182,584, incorporated by reference) provide a relatively fixed lower riser section, buoyed at a submerged location to avoid ocean turbulence and a compliant section connected to a production vessel. Considerable force must be withstood at the point of connecting the buoyed riser at the marine base. Considering the many tons of vertical force and deflection of the riser due to ocean currents, a direct load-bearing mechanical connection between the production riser section and manifold chamber has been impractical.
It is an object of the present invention to provide a reliable subsea riser manifold system capable of handling multi-well fluids and withstanding the rigors of a large riser connection. According to the present invention, subsea riser manifold system is provided for handling marine well fluids from multiple subsea wells and transmitting the well fluids through a marine production riser.
A marine floor base template having a support structure is adapted to support a manifold chamber. A plurality of pile guides are connected to the template for fixing the template to the marine floor, and a sealed manifold chamber hull is mounted on the template between the pile guides. This chamber hull encloses manifold means for operatively connecting the subsea wells to the production riser. The improved manifold system includes a structural spanning support member extending over the manifold chamber hull for receiving and supporting the marine production riser. This spanning member has an upper riser-receiving platform portion and structural arms connected between the platform portion and the pile guides, whereby production riser load is transmitted directly to the pile guides through the spanning member.
The preferred manifold chamber hull comprises a fluid-tight horizontal cylindrical pressure vessel and means for maintaining a low pressure atmosphere therein. Advantageously, the manifold spanning member has a pair of spanning arms on opposite sides of the manifold chamber, each arm extending outwardly and downwardly from the platform portion in a spider configuration, connecting the riser in load-bearing relationship to the pile supports. Ordinarily the platform portion is vertically spaced from the manifold chamber hull and has at least one access opening to permit connection of production riser conduit through the manifold chamber.
These and other advantages and features will be understood from the following description and in the drawing.